Electroluminescent lamp



Dec. 26, 1961 G. H. BOUCHARD ETAL 3,014,813

ELECTROLUMINESCENT LAMP Original Filed Jan, 17, 1955 INVENTORS GEORGE/iBOUCH/J/PD BY JOSPH A. DOMBROWSK/ AT TOR/V5 Y United States Patent663,474 3 Claims. (Cl. 11733.5)

This invention relates to electroluminescent lamps and to methods ofmanufacturing them. In particular, it relates to lamps in which thephosphor is embedded in a dielectric material.

Such lamps have previously had a coating of phosphor and dielectricapplied by spraying. We find, however, that a more uniform coating ofgreater brightness and higher dielectric strength can be applied bysettling the coating from a temporary suspension of the fine particlesof phosphor and dielectric in a liquid medium. The liquid should be onein which the particles will not readily dissolve, that is the phosphorand dielectric particles should not be readily soluble in the liquidused. Powdered ceramic is especially useful as the dielectric material.

An advantage of our invention is that the electroluminescent layerproduced is uniform and accordingly free from mottled portions.Dielectric breakdown is often caused by very small pinholes extendingthrough the layer of phosphor and dielectric. We find that suchbreakdown can be greatly reduced by depositing the phosphor-dielectriccoating in at least two layers, one over the other, partly because theprobability of any pinholes in one layer lining up with those in theother layer to produce a direct path from one electrode to the other isvery small. This is especially true when each layer is deposited undersomewhat different conditions.

Lamps can be made with a single phosphor-dielectric coating by usingextreme care to keep all materials free from lumps, lint, room-dust, andthe like, and by carefully controlling the particle size so that exactlythe right proportions of ceramic and phosphor are settled down together.Such extreme care and cleanliness is diflicult and expensive to maintainin production.

However, we have found that by settling the material in two coatings orlayers, a satisfactory device can be made without the need for suchextreme precautions.

For example, we can apply a transparent conductive film to a porcelainenamel surface to act as one electrode, and then apply the firstphosphor-dielectric coat over the conductive film and heat it at a lowtemperature such as 1150 F. to sinter the coating without damaging thefilm. The comparatively low temperature of.the firing reduces anypossible damage to the phosphor during the first heating. It burns outany contaminants that may be present, without causing any holes andwithout disturbing the coating. The second coat is generally applied assoon as the first coat has cooled, but we find that the surface of thelatter will not be affected by the small amount of contamination pickedup even if the coating is allowed to stand overnight. The second coat isfired at a higher temperature.

In our process, there is a selective settling by particle size, thelarger part of the phosphor settling first with the larger ceramicparticles. The finer phosphor particles then settle with themedium-sized ceramic particles and finally a layer of very fine glassparticles settles, nearly free of phosphor. With two thin coats there isa duplication of these layers, which appears to give a lamp of highdielectric strength for a given weight or thickness of coating.

Other objects, features and advantages of the inven- 3,014,13 PatentedDec. 26, 1961 FIGURE 2 is a schematic view of the two superposedphosphor-dielectric layers.

In FIGURE 1, the metal backing plate 1 carries a porcelain enamelcoating 2 such as is known in the art, the enamel preferably beingwhite, and which can have any convenient thickness, for example 10 mils.A transparent conductive coating 3 of stannous chloride or the like ispreferably applied as in the copending application Serial No. 365,617,filed July 2, 1953 by Richard M. Rulon, over the enamel. The first andsecond coatings 4, 5 of phosphor and ceramic are over the conductivecoating 3, and another transparent conductive coating 6, applied asbefore, over the coating 5. Provision for connections to an electricpower line can be made as in said Rulon application.

In FIGURE 2, the phosphor-dielectric layers 4, 5 are shownschematically. The layers 4, 5 contain a ceramic material 7, 9 with thephosphor particles 8, 10 mostly at the bottom of each layer. The figureis schematic, and while most of the phosphor particles are near thebottom of the layer, not all of them are in that position. The holes 11,11 in the top layer 5 are seen to be out of register with the holes 12,12, 12 of the bottom layer 4, so that there is no direct breakdown paththrough both layers 4, 5, in series. Moreover, the portion of nearlyclear glass, almost free from phosphor, at the top of each layer, alsoimproves the dielectric strength.

In making a lamp according to one embodiment of the invention, a clearmetal tank large enough to hold one or more lamps of the desired size ina horizontal position is vacuum-cleaned and flushed with alcohol. Theenamel coating 2 on metal backing plate 1, is brushed with asoft-bristled brush to remove dust and any abrasive material, and placedon a rack at the bottom of the tank, preferably spaced a short distance,for example 4 inch, above the bottom of the tank.

A suspension of ceramic and phosphor in a suitable liquid is then pouredinto the tank through a strainer. Unnecessary turbulence and splashingof the material on the coating 2 should be avoided.

A cover is placed on the tank and the phosphor-dielectric material isallowed to settle out for about 15 minutes. The remaining suspension isthen drained slowly from the tank and the coating 4 allowed to partiallydry while the tank is covered,-for example for about five minutes, if instill air at room temperature. Under other conditions, the time may bedifferent, but in any case it should be long enough to dry the coatingsutliciently to allow the removal of the piece from the tank withoutproducing flow lines in the coating, due to running of any undriedmaterial. If the amount of phosphor and ceramic in the suspension isincreased, the time for deposition of a coating can be reduced to 1%.minutes or less. After partial drying, the cover is removed and thecoating completely dried.

The coating is then fired in a furnace at about 1150 F.

The coated piece is then returned to the tank and the process repeated.The resultant coating should be snowywhite and very smooth and even. Thedried piece is then fired at 1250 F.

The coating suspension is made in a S-quart ball mill with 3000 grams ofpebbles. About 2000 grams of ceof methyl alcohol and the rinsings addedto the jar through the 200-mesh sieve. The suspension in the jar is thenrolled in its jar (which has been sealed) for 20 minutes before beingused.

To one part of the above suspension is added a mixture of 210 parts of a50% methyl and 50% isopropyl alcohol mixture. If a tank with ahorizontal cross-section of 26 inches by 26 inches is used, 7500 cc. ofmethyl alcohol and 7500 cc. of isopropyl alcohol will be used with 70cc. of the milled suspension. This will fill the tank to about 1% inchesabove piece to be coated, which will be lying horizontally in the tank,about inch from the bottom.

Although other suspending liquids can be used, the above mixture ofmethyl and isopropyl alcohols has been found very etfective in producinga uniform unmottled coating of high brightness and dielectric strength.The liquid used should preferably have a specific gravity, viscosity andvolatility about the same as that of the mixture above. If the specificgravity and viscosity are too low, the particles will settle out toofast, without the selective action which has proven beneficial. On theother hand if the volatility is too great, drying will occur' too fast,before the coating has time to smooth out. Isopropyl alcohol alone, ormethyl alcohol alone, can be used but is not as satisfactory as themixture given.

Toluol or water can also be used, but the coating will be less uniform.With water, the drying rate should be increased by forced ventilation toprevent streaks in the coating. Such streaks occur with slow drying.

The phosphor used can be any suitable electroluminescent phosphor, forexample the phosphors of copending applications Serial 'Nos. 230,711and. 230,712, filed on June 8, 1951 respectively, by Keith H. Butler,and by Keith H. Butler and Horace Homer, and now US. Patent Nos.2,772,242 and 2,745,811, respectively.

The ceramic frit used can be of the kind shown in copending applicationSerial No. 365,617, filed July 2, 1953 by Richard M. Rulon. One suchfrit can, for example, contain 3.9% CaO, 4.8% BaO, 27.2% ZnO, 21.8% SiOB303, N330, R203, the percentage being taken by weight. In the lastnamed compound, R has been used as a generic term to include one or moremetals whose oxides can exist in the imperial form R Lead should not bepresent in the material in any substantial amount in a form which canreact with the phosphor to form a black, light absorbing layer such aslead sulfide.

To protect the device from deterioration in high humidities, a glaze ofplain glass frit, without phosphor, is generally placed over thetransparent conductive film 6 and around the edges of the device to sealthe same. This glaze can be applied in the same manner as the coatingcontaining phosphor, except that the phosphor is omitted. For example,2000 grams of the glass frit can be added to the ball mill with 1500 cc.of isopropyl alcohol and rolled in the mill for about two hours, afterwhich it is poured off into a jar and about 650 cc. of isopropyl alcoholadded to the mill to rinse the latter. The rinsings are added to thesuspension in the jar, the latter sealed and then rolled for aboutfifteen minutes. This suspension can then be applied by settling andfired as with the second phosphor-ceramic coating 5.

For complete humidity protection, a layer of low viscosity resin, forexample an alkyd styrene urea copolymer, is deposited over the glaze,having a thickness of about two mils. This will fill any pores in theglaze and complete the humidity seal.

A composite seal of that type is more effective than an all-glass sealor an all-resin seal.

In order for the phosphor particles to settle out first,

they should ordinarily have either higher specific gravity The ceramicpowder has sometimes been referred to as.

ceramic frit or glass frit in the foregoing description.

This application is a division of our copending United States patentapplication Serial No. 482,126, filed on January 17, 1955 for anElectroluminescent Lamp, and issued on February 25, 1958 as UnitedStates Patent 2,824,- 992.

What we claim is:

1. The process of coating a base piece with an electroluminescent layerof predetermined thickness, said process comprising: mixing powderedzinc sulfide phosphor particles of from 10 to 20 microns size andpowdered ceramic particles of between one-third and one-tenth the sizeof the phosphor particles with a mixture of methyl and isopropylalcohol, and immersing the base piece in it for a time sufficient todeposit on it a coating of predetermined thickness.

2. The process of coating a base piece with electroluminescent phosphor,said base plate having a conducting film thereon, said processcomprising: settling a coating of phosphor particles and ceramicparticles onto said base piece from a temporary suspension in a mixtureof methyl and isopropyl alcohols, the phosphor particles being from 10to 20 microns in size and the ceramic particles being between one-thirdand one-tenth the size of the phosphor particles, drying said coating tofree it from said mixture of methyl and isopropyl alcohols, firing saidcoating at a temperature of about 1150 F. and high enough to sinter saidceramic without affecting the conducting film, settling another suchcoating over the first coating, and then firing at a higher temperatureof 1250" F.

3. The process of coating a base piece with electroluminescent phosphor,said process comprising: settling a coating of said phosphor particlesand ceramic particles onto said base piece from a temporary suspensionin a mixture of methyl and isopropyl alcohols, the phosphor particlesbeing from 10 to 20 microns in size and the ceramic particles beingbetween one-third and one-tenth the size of the phosphor particles,drying said coating to free it from said mixture of methyl and isopropylalcohols, firing said coating at a temperature of about 1150 F. and highenough to sinter said ceramic, settling another such coating over thefirst coating, and then firing at a higher temperature of 1250 F.

References Cited in the file of this patent FOREIGN PATENTS 118,070Australia Feb. 10, 1944 617,099 Great Britain Feb. 1, 1949 721,513 GreatBritain Jan. 5, 1955

1. THE PROCESS OF COATING A BASE PIECE WITH AN ELECTROLUMINESCENT LAYEROF PREDETERMINED THICKNESS, SAID PROCESS COMPRISING: MIXING POWDEREDZINC SULFIDE PHOSPHOR PARTICLES OF FROM 10 TO 20 MICRONS SIZE ANDPOWDERED CERAMIC PARTICLES OF BETWEEN ONE-THIRD AND ONE-TENTH THE SIZEOF THE PHOSPHOR PARTICLES WITH A MIXTURE OF METHYL AND ISOPROPYLALCOHOL, AND IMMERSING THE BASE PIECE IN IT FOR A TIME SUFFICIENT TODEPOSIT ON IT A COATING OF PREDETERMINED THICKNESS.